WiFi security is really bad. I would be wary of joining any WiFi network out in public. Especially if it says free, and even more so if you were heading to Sao Paulo for the World Cup. The gist of the article here is that WiFi networks have bad security and so do apps.

What this world really needs is WiFi everywhere, because it's proven to be a secure way to communicate with the internet. Oh wait... This is a good article that goes into more technical detail and how you device can be pwned connecting to a public WiFi network.